The set-in-stone rule for changing your oil is no longer 3,000 miles. What you should do is check the owner's manual to see the recommended mileage between oil changes. These days, many cars can go as long as 8-10,000 miles between changes or around a year. If you have a more odd schedule, put a small piece of tape somewhere in the engine and write down every time your oil has been changed. That way, you can easily check, and you'll always know when it's time to switch out. The last thing you want is gooky build-up.